1. Field of the Invention:
The present invention relates generally to communication systems, and more particularly, to transducers and transduction methods for reproducing wide audio bandwidth sound using an ultrasonic carrier within a communication system.
2. Background Information:
Communication systems typically operate with transducers that convert audio acoustic signals into electrical signals, and vice versa. The audio acoustic signals are airborne sound pressure waves having frequencies within the bandwidth detectable by the human ear (acoustic signals having frequencies between approximately 20 Hertz (Hz) to 20 kiloHertz (kHz)). Ultrasonic acoustic signals are not output from typical audio circuits because these signals possess frequencies outside the bandwidth detectable by the human ear, and produce inaudible sound pressure waves.
However, communication systems are known wherein ultrasonic signals are used as carrier -signals in the production of audio acoustic signals. These systems typically rely on either: (1) the non-linearities of air to demodulate an audio modulated ultrasonic carrier signal; or (2) rely on bone conduction of ultrasonic signals to create the sensation of audio signals. As such, these systems are ill-suited for or even unable to produce high fidelity sound.
For example, a document entitled xe2x80x9cNorris Acoustical Heterodyne(trademark) Technology and HyperSonic(trademark) Soundxe2x80x9d (Jul. 26, 1997) by Elwood G. Norris of American Technology Group (California) describes a distributed speaker system wherein the ultrasound transducer superposes an audible signal on an ultrasonic signal of such intensity that airborne audible sound pressure waves, detectable by the human ear, are created. By superposing audible frequencies in the 20 Hz to 20 KHz bandwidth onto an ultrasonic tone, the transducer can be designed to provide uniform audio over a frequency range which constitutes a much smaller percentage of the transducer""s center frequency. That is, without the use of an ultrasonic carrier, the total frequency range of the audible bandwidth (i.e., approximately 20 kHz) divided by the lowest frequency in the bandwidth (20 Hz) constitutes a percentage frequency shift from the lowest frequency (20 Hz) to the highest frequency (20 kHz) of 20 kHz/20 Hz, or 100,000%. By superposing this 20 kHz band on an ultrasound carrier in the 200 kHz range, the percentage frequency shift reduces to 20 kHz/200 kHz, or 10%, such that the transducer can be more effectively designed. However, this speaker system requires the use of high intensity output signals because it relies upon the non-linearities of air to demodulate the ultrasonic signals into audible acoustic signals. Thus, efficiencies which are gained in the transducer design are lost in the demodulation.
The Norris document describes transmitting two ultrasound wave trains each having a tone of sufficiently high amplitude that when introduced to the nonlinearity of air in the room produce two xe2x80x9ccombinationxe2x80x9d tones corresponding to the sum and difference of the two original ultrasonic tones. For example, if two ultrasonic tones of 200 kHz and 201 kHz were emitted from the ultrasound transducer into air with sufficient energy, a sum tone of 401 kHz and a difference tone of 1 kHz would result, that latter being within the range of human hearing. The distributed speaker system thus relies on the non-linearity of air and the resultant difference tone to produce an audio acoustic signal having pressure waves that can be detected by listeners.
A document entitled xe2x80x9cIn The Audio Spotlightxe2x80x94A Sonar Technique Allows Loudspeakers To Deliver Focused Sound Beamsxe2x80x9d, Scientific American, October. 1998, pp. 40-41, describes the demodulation of audio tones from ultrasonic waves using the non-linearities of air, and discusses the work of Norris. This document mentions the distortion which occurs at low frequencies of the audible bandwidth when audible tones are produced from ultrasonic waves using the non-linearities of air (i.e., poor bass). This document suggests that using the non-linearities of air to demodulate an ultrasonic carrier to produce sonic energy compromises the ability to achieve high fidelity, wide audio bandwidth sound having a full, bass response. Such a compromised ability would be unacceptable for high fidelity communications.
The lack of audio bass in systems which rely on the non-linearity of air to produce sonic energy occurs because the ultrasound-to-audio-sound transformation is essentially constant volume displacement. That is, the volume of air moved is essentially the same no matter what sonic frequency is being converted. However, the human ear is essentially constant pressure. That is, to hear a constant loudness over a range of audible frequencies requires that those frequencies be presented at the same pressure (also called Sound Pressure Level or SPL). In air, volume displacement and pressure are related, as a function of frequency, as P=V*f, where P is pressure, V is volume displacement and f is the frequency. Thus, to maintain constant pressure (i.e. SPL) as the audible sonic frequency is reduced, the volume displacement must be proportionally increased. Since the Norris technique does not inherently increase the volume displacement at lower audio frequencies, a proportional increase in the ultrasound drive signal must be used. This is called equalization, and it results in a need for very large drive signals to reproduce flat low bass audio frequencies. However, boosting the lower frequencies can not sufficiently compensate the loss without causing distortion when the transducer is driven hard.
Another document entitled xe2x80x9cAudio Sound Reproduction Based On Nonlinear Interaction of Acoustic Wavesxe2x80x9d by Dong Weiguo and Wu Qunli, J. Audio Eng. Soc., Vol. 47, No. 7/8 1999 July/August also describes the nonlinear interaction of two finite-amplitude sound waves of different ultrasonic frequencies in air to produce audible sound waves whose frequencies correspond to the difference of the primary waves. However, this document describes the difficulty in relying on non-linearities of air, as opposed to fluids, to exploit this effect. This difficulty is due to the high absorption of acoustic waves in air and the small non-linearity parameters of air. As experimentally confirmed by Weiguo, the audio frequency SPL output is indeed proportional to frequency; that is, for a constant ultrasound input, the audio output was 34 dB lower at 20 Hz than at 1 kHz. Furthermore, this inherent effect is the same whether the two ultrasonic frequencies are produced from two different ultrasonic transducers or from a single transducer. Thus, like the Scientific American document, this document suggests that using the non-linearities of air to demodulate an ultrasonic carrier compromises the bass response, a result which would be unacceptable for high fidelity communications.
Norris, in U.S. Pat. No. 5,889,870, entitled Acoustic Heterodyne Device and Method, describes headsets and hearing aids based upon his acoustical heterodyne method. However, both devices rely upon the non-linearities of the air transmission medium. For example, they rely on the non-linearities of air within the ear canal itself to create air borne audible acoustic waves that are subsequently detected by the normal acoustic hearing process of the ear. Furthermore, the Norris patent discloses that this process requires a resonant cavity, and that the ear canal""s natural resonance properties provide that necessary element (col. 15, lines 14-27). There is no mention of a system which does not require the nonlinearities of air nor one which works without the xe2x80x9cbroadly resonant cavityxe2x80x9d.
The use of ultrasonic signals is also described in a document entitled xe2x80x9cHuman Ultrasonic Speech Perceptionxe2x80x9d by Martin L. Lenhardt et al, Science 1991: 253: 82-85. However, rather than relying on the non-linearities of air to demodulate an ultrasonic carrier, this document is directed to use of bone-conducted ultrasonic signals. The bone conducted ultrasonic signals are asserted to have potential as an alternative communication channel in the rehabilitation of hearing disorders.
It is unclear from the Lenhardt document exactly how the ultrasonic signals are converted into detectable sensations. However, Lenhardt discloses tests performed using the two sidebands of a modulated ultrasonic signal. The two sidebands constitute two different ultrasonic frequencies generated using a dual side band (DSB) suppressed carrier modulation method, and are received via bone conduction by the inner ear. Non-linearities of the bone conductor mechanism are presumably used to detect a difference between the two ultrasound sidebands which are spaced at twice the input frequency. Since the two sidebands are spaced from one another by twice the audio frequency used to modulate the ultrasonic carrier, the detectable audio frequencies would be doubled and the natural spacing of speech components would not be preserved. The double sideband suppressed carrier modulation technique diminishes the intelligibility of speech, and renders the Lenhardt approach unsuitable for high fidelity sound.
The reliance of the Lenhardt approach on the non-linearities of the bone conduction mechanism to produce audible sensations is supported in a document by Staab, et al. entitled xe2x80x9cAudible Ultrasound For Profound Lossesxe2x80x9d, The Hearing Review, Febuary 1998, pages 28-36, which cites the Lenhardt et al document and its disclosed use of an amplitude-modulated, suppressed carrier (double sideband modulated) technique, with speech superposed on the carrier. Page 30 of The Hearing Review document describes a HiSonic(trademark) hearing aid device developed by Hearing Innovations Inc. of Tucson, Ariz. as an outgrowth of the Lenhardt et al technology. The Hearing Review document indicates on page 30 that in a test where a piezoelectric bone conduction driver was applied directly to the mastoid of the skull, no audio signal was measurable using a force transducer on the mastoid or a probe microphone in the ear canal. Thus, it is concluded that the audible sensations detected did not come from any airborne audio signal but must have resulted from some internal non-linearity in the bone conduction path.
The Lenhardt document does not disclose the use of a transducer with an impedance matched to air, and therefore it is incapable of directing inaudible, airborne ultrasonic signals down the ear canal of a user to produce sound that is detectable by the user. Page 36 of The Hearing Review document suggests that the bone conducted ultrasound may directly stimulate a nerve, stimulate the cochlea, or stimulate a secondary auditory pathway. However, the use of bone conduction, coupled with the use of a double sideband suppressed carrier, compromises the fidelity of sound achievable with the device.
In the Lenhardt, et. al. U.S. Pat. Nos. 4,982,434 and 5,047,994, both entitled Supersonic Bone Conduction Hearing Aid and Method, it is disclosed that the bone conduction method is based on a system of hearing quite distinct from normal hearing based on air conduction. (""994, col. 1, lines 61-63). Furthermore, in the ""434 patent at col. 2, lines 28-38 Lenhardt discloses that his method relies upon direct bone transmission to the saccule and this enables hearing to be maintained via a system independent of air conduction and the inner ear, and utilizes frequencies that are perceived by the saccule and not by the inner ear. Thus, Lenhardt""s hearing aid device is based upon the ultrasonic sensitivity of a non-hearing organ.
Additionally, the signal from Lenhardt""s bone-conduction ultrasound transducer is coupled to the mastoid region of the head by, for example, applying significant pressure with the transducer or with coupling gel or both. This is because the transducer""s acoustic impedance is matched to the impedance of the bone so that good signal transfer can be obtained. The impedance of air is many orders of magnitude lower, so that even a slight separation of the transducer from the head would produce a nearly total dropout of the signal. Thus, the Lenhardt""s approach is inconvenient or even painful, especially for long wearing periods.
Furthermore, Lenhardt discusses that his method suffers from an expansion of the Just Noticeable Differences (JND) of frequency. Lenhardt""s device therefore includes a frequency expander, the purpose of which is to stretch the spacing of the audio frequencies so that the modulation sidebands can be sensed as separate frequencies (""434, col. 4, line 50 through col. 5, line 2).
Shannon, et. al. in U. S. Pat. No. 5,285,499 entitled Ultrasonic Frequency Expansion Processor, further describes this JND problem and references the Lenhardt, et. al. patents. In this Shannon patent, a method for accomplishing frequency expansion is disclosed, based upon digital signal processing methods and specifically utilizing pitch shift processing combined with single-sideband upconversion to generate the bone conduction drive signal. Although the use of digital pitch shifting is disclosed in this document, it is disclosed for overcoming the JND bone conduction problem by expanding the frequencies of the incoming audio signal prior to modulation of the ultrasound signal.
In summary, known communications systems do use inaudible ultrasonic signals to produce sensations that are detectable as sound by the human ear. However, because these systems either rely on the non-linearities of air to demodulate the ultrasound, or rely on bone conduction of ultrasonic signals to create the sensation of audio signals, they cannot provide high fidelity audible sound. In addition, the bone-conduction method is, at best, very uncomfortable in use.
The present invention is directed to a communication system wherein ultrasonic signals can be used as carriers to efficiently produce high fidelity, wide audio bandwidth sound. Exemplary embodiments rely on the airborne transport of an inaudible ultrasonic carrier directly into the hearing mechanism of a user, such that the known non-linearities within the ear itself can be exploited to demodulate the ultrasonic carrier without producing audible sounds at the input to the user""s ear. The non-linearities of the ear itself, in conjunction with the human brain""s perception of audible frequencies generated in response to ultrasonic stimulation, are relied upon to detect audio information.
The ultrasound-to-audio-sound conversion in the middle and/or inner ear does not require creation of audible sonic pressure waves in the air, but rather directly converts ultrasound difference frequency pressure into audible pressure. Thus, this conversion is constant pressure and all frequencies of the audio bandwidth (including low frequency bass signals) are produced with comparable sound intensity.
An exemplary communication device of the present invention comprises: means for establishing an ultrasonic signal; means for modulating the ultrasonic signal with an audio signal to produce a modulated ultrasonic signal at an output; and means for mounting the output in proximity to a human ear canal at a location where a hearing mechanism associated with the ear canal receives the audio signal as inaudible airborne ultrasonic acoustic energy.
Modulation techniques are selected which exploit the direct introduction of the modulated ultrasonic signal into the user""s ear. For example, modulation techniques such as double sideband with carrier, carrier-plus-single-sideband, and pitch shifting techniques (e.g., pitch shifting techniques combined with modulation which suppresses the carrier) are used.
Exemplary embodiments of the present invention provide significant advantages. For example, where a communication device configured in accordance with the present invention is used as the speaker transducer of an earpiece worn by the user, the output from the speaker transducer can be directed toward the tympanic membrane of the user""s ear and, due to the non-linearities of the ear itself, result in perceptible sound to the user. However, since audio acoustic energy is not produced in the air, audio acoustic sound is not radiated from the user""s ear. As such, others in the vicinity of the user will hear no sound from the earpiece, thereby providing secure secret communication. This is particularly useful in surveillance or covert operations. Other people near the user will not be annoyed by incoming signal sounds produced by the communication device, even in the most quiet of environments, because audible sounds are not supplied to the user""s outer ear.
Other advantages include that this non-contact method provides extreme comfort for long-term usage. This is due to at least two factors: first there is no contact pressure or messy coupling gel required as for the bone-conduction methods and second, because the ear canal is not occluded, as is the case with many earbud and hearing aid sound delivery systems, there is complete lack of the occlusion effect.
Exemplary embodiments can be configured of small size and light weight, such that they are comfortable to wear and yet still achieve the benefits of secrecy and quiet operation. Such features are particularly advantageous where the speaker transducer is included in an earpiece situated within the ear canal such that a portion of the canal remains open (i.e., an open canal earpiece). With an open canal earpiece, the user can, in addition to hearing output signals from the earpiece, also comfortably hear ambient sound in a vicinity of the user.